S-Bond active solders were instrumental in providing a seal for the Transition ring that feeds liquid oxygen into Vulcan Centaur’s main engine. These seals have undergone rigorous cryogenic testing resulting in ULA’s use of this technology.
Contact Us for more information on how S-Bond solders can solve your bonding challenge.
In sputter targets, the deposition/target materials is sputtered (high energy ion impacts displace a target’s atoms) onto an opposite surface to form a functional coating or other element layer in a computer chip, TV screen or other semiconductor device. The high energy impact of the deposited films require “sputter targets” to be cooled to keep the target material layer from melting. To cool the target, water is circulated in an aluminum or copper base plate. S-Bond active solders can bond all metals and most ceramics that are commercially used to produce sputtered films. Active solders can bond these sputter targets to either Copper or Aluminum, bonding them at low temperatures (which minimizes thermal expansion mismatch stresses), without flux contamination/entrapment (which will contaminate the sputtering plasma), and with excellent thermal conductivity (a metallic bond with nearly no voids).
For probes and sensors, S-Bond active solders can bond silicon or carbide/carbon based devices (MEMS or other semiconductor based probes) directly to metal and metallic conductor leads. The low temperature active solders, when joining semi-conductor devices, impart low residual stresses and the joints are electrically and thermally conductive… good for signal transmittance and cooling.
Piezoceramics such as Pb-Zirconates (PZT) that impart force/small displacement or create ultrasonic pulses, can be bonded direct to metals with active solders. Bonding is accomplished bonded below their curie points with acoustically sound interfaces that can transmit sound effectively. Such piezoceramic based sensors and actuators are used in accurately measuring gas flow and can be used on gas control in MOCVD processes used to deposit and etch computer chips. S-Bond active solders can wet and adhere to most piezoceramics all without pre-plating and chemical fluxes… as such they are finding excellent application in probes and sensors used on semiconductor processing.
Wafers (silicon or other) are placed into energetic plasmas and other beams to deposit then etch a complicated surface morphology in layers to create semiconductor based chips. The high heat energy into the wafer needs to be removed through water cooled wafer handling devices such as the pedestals the wafer sits on in their processing chambers. If not cooled, the interdiffusion of the fabricated on the wafer. With the latest 300mm diameter wafer technology over a $1M work of chips can be on a wafer.
The high energy levels used in semiconductor processing requires well cooled and reliable handling equipment. S-Bond active solders can intimately join copper and aluminum as well as other thermal management materials such as AlSiC and pyrolytic graphite.
If you would like to see how S-Bond active solders can improve your semiconductor processes and handling and measuring equipment, please Contact US.
Sensors and actuators are a growing commercial market with the Internet of Things (IOT) and the interest to remotely monitor and control many devices.
S-Bond® active solders are finding more application in sensors and actuator devices due to their use of dissimilar materials, including metals, intermetallics, ceramics, composites and glasses which need to be joined. S-Bond® active solders are unique in that they can join such materials, without flux or plating, at low temperatures and with excellent conductivity (both electrical and thermal).
With Ce, Ga and Ti additions to solder filler metals, S-Bond® solders can bond direct to oxides, nitrides and carbides that have formed on metal surfaces, directly. On aluminum and copper the Ga and Ce interact with the oxides that form on these metals then the Sn and Ag constituents form metallurgical intermetallic compounds (IMC’s) that chemically bind the solder to the aluminum or copper base metal. With the active S-Bond® solders’ ability to wet, adhere and join such a diverse set of materials, the S-Bond® alloys find wide application in sensors and actuators that employ a diverse array of materials and in dissimilar material joints… These joints have many requirements, depending on the application, these requirements include…
Bond strengths high enough for the application
Low temperature joining
Accommodation of CTE mismatch strain
Sensors and Actuators that S-Bond® is currently specified in includes…
Piezo ceramic (PZT) – Ultrasonic Gas Flow sensors; PZT ceramic disks are S-Bond® soldered to stainless steel housings that transmit and receive u/s sound pressure waves. The transmit sensor with bonded PZT piezo ceramic disk sends u/s waves into a passing gas flow and a receive sensor with bonded PZT receives and converts the sound waves… with a shift in frequency known as the Doppler effect, can relate the frequency shift to the mass flow of the passing gas stream. In these sensors, the piezo ceramic disk needs to be intimately bonded with no voids to create an acoustically “hard” transmitting bond interface, joining the ceramic to metal below the curie temperatures of 250 C. S-Bond® 220 alloys are being used to make these reliable and acoustically sound interfaces.
MEMS Pressure Gages; Silicon based MEMS devices us Si-dies and incorporate circuitry to use the Si as part of the measurement. In the case of pressure measurements, thin diaphragms of Si are created and strain gage circuitry is deposited using lithography to complete the sensor… the challenge then was so seal the Si-sensor die to a metallic pressure housing that is installed onto the component needing a pressure sensor. S-Bond® active solders can join Si direct to metals and can create a hermetic joint, creating a seal between the Si-MEMS pressure sensor and the mounting housing.
Graphite Electrodes / Water Conductivity Sensors; S-Bond® active solders are being used to join graphite to electrical leads for use in Anode/Cathode systems for making excellent electrical solder connections with the use of flux or pre-plating.
Sapphire – Optical Sensors; Sapphire is single crystal aluminum oxide that is very hard/scratch resistant and also transits optical signals in a specific spectrum. As sapphire is an excellent “window” material for many optical signals. For example in Gamma Ray Detection, NaCl single crystal creates photon (light) output proportional to the impinging gamma ray radiation intensity. The NaCl crystal will degrade/dissolve in contact with air, to the crystal is housed behind a sapphire window which is S-Bond® active solder sealed (hermetic/He leak free) to a titanium tube to create a sealed environment that the gamma rays can penetrate. Optical detectors are then mounted in front of the sapphire window, outside the S-Bond® sealed enclosure.
Insulators /Radar Sensors; Printed 3-D circuits are being made to generate / receive radar signals. These circuits are built through ceramic layers that form a ceramic backbone to the sensors’ circuitry. S-Bond® active solders have been used to bond the edges of this ceramic backbone of the sensor and seals it from the environment.
Magnet Assemby – Actuators; Magnetic actuators are used to move valves, switches and other devices dependent on precise and reliable stroke based motion. Such magnetic actuators are using high force as CoSm based magnets. These magnets will form a strong and specific magnet fields. In one actuator design, the actuator “rod” runs on the magnet assemblies’ magnetic axis. To assure optimal actuator lineal translation, the actuator’s central push rod could not be magnetic, so it a ceramic rod was selected. In this actuator, S-Bond® active solders have been used to bond the central ceramic rod to the magnetic core of the actuator.
As presented here, S-Bond® solders are being applied in a growing range of sensors and actuators. If you would like to take advantage of S-Bond® solders unique capabilities to join dissimilar materials in your sensors and actuators, please Contact Us.
S-Bond® active solders enable graphite bonding and the joining of other carbon based materials to each other and to most metals within the constraints of thermal expansion mismatch. S-Bond® alloys have active elements such as titanium and cerium added to Sn-Ag, Sn-In-Ag, and Sn-Bi alloys to create a solder that can be reacted directly with the carbon surfaces prior to bonding using specialized S-Bond® treatments for solder joining. Reliable joints have been made between graphite and carbon based materials with all metals including steel, stainless steels, titanium, nickel alloys, copper and aluminum alloys.
In high power density electronics, there is a need to rapidly spread and dissipate heat generated by the high frequency operations in the electronics. In order to improve the heat dissipation capacity of graphite based materials, Applied Nanotech developed a new passive thermal management material, CarbAl™, which is a carbon-based material with a unique combination of low density, high thermal diffusivity, and low coefficient of thermal expansion based on Figure 1.
Figure 1. Picture of the CarbAl-G high thermal diffusivity graphite composite.
Applied Nanotech reports that CarbAl™ has a density of 1.75 g/cm3 compared to 2.7 g/cm3 for aluminum and 8.9 g/cm3 for copper. While copper has a slightly higher thermal conductivity than CarbAl™, 390 W/mK compared to 350 W/mK, CarbAl’s thermal diffusivity is approximately 2.9 cm2/sec compared to 0.84 cm2/sec for aluminum and 1.12 cm2/sec for copper.
S-Bond Technologies and Applied Nanotech have collaborated to make heat spreaders with CarbAl-G cores combined with copper and aluminum claddings to make the heat spreaders that are more robust and able to be fabricated to support a high density of high power electronic devices, yet be mounted in standard “PC” card configurations as seen in Figure 2.
Figure 2. Copper and Aluminum clad CarbAl-G circuit boards.
These clad CarbAl-G cored boards have utilized active S-Bond® solder layers to intimately bond and thermally connect the thin copper or aluminum claddings to the lightweight, high thermal diffusivity CarbAl-G composite sheets.
S-Bond® CarbAl-G bonding and joining was thermally activated using S-Bond Technologies proprietary process, which prepared the CarbAl-G surfaces and developed a chemical bond to the surface, through reactions of the active elements in S-Bond® alloy to the graphite in CarbAl-G. These joints start with processing the graphite/carbon surfaces at elevated temperatures in a protective atmosphere furnace with S-Bond® alloy placed on the graphite-carbon surfaces to be joined. At these elevated temperatures, the active elements in S-Bond® (Ti, Ce, etc.) react with the ceramic to develop a chemical bond. After the CarbAl-G is prepared / S-Bond® metallized, the CarbAl-G is then S-Bond® soldered to the aluminum or copper sheets to for a metal clad CarbAl-G composite plate that can then be machined into a heat sink plate to which high power electronic devices be mounted.
The S-Bond® solder joints produced:
Are ductile, based on Sn-Ag or Sn-In alloys
Exceed the strength of the CarbAl-G
Are thermally conductive, with S-Bond® alloys having k = 50 W/(m-K)
Are metallic and this electrically conductive with a metallurgical bond
S-Bond Technologies has developed extensive experience in active, S-Bond® solder joining of graphite, carbon and carbide to metals. Contact Us to evaluate our joining solutions for your graphite joining applications. For more information on CarbAl-G, please contact Applied Nanotech. Inc
Ronald Smith S-Bond Technologies Inc., Hatfield, PA,
Lawrence W. Shacklette, Michael R. Lange, James C. Beachboard, Harris Corp., Melbourne, FL
and Donna L. Gerrity E&S Consulting Inc., St. Augustine, FL
Packaging of optical devices often requires the need for creating strong bonds between metal and silica. The most convenient and cost effective approach would be to directly solder to both silica and metal without requiring pre-metallization of the silica. Soldering to oxides and oxidized surfaces has been accomplished with various solders containing metals with strong affinity for oxygen, known as “active solders”.
S-Bond Technologies worked with Harris Corporation to understand S-Bond® 140 active solders, based upon a tin-bismuth eutectic with activating additives of cerium, gallium, and titanium, to produce seals between metals and silica. Titanium and cerium are energetically capable of competing for the oxygen in silica, and are therefore capable of reducing or forming mixed oxides with silica under appropriate conditions. The bond between such an “activated” solder and high purity fused silica (HPFS) has been characterized by Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS). Two variations of solder produced by S-Bond Technologies, S-Bond®140 and S-Bond®140 M1 were bonded to silica using a fluxless ultrasonic technique.
Figure 1 illustrates the ultrasonic soldering process where the resultant cavitation of the molten solder layer continually disrupts the oxides forming on the molten solder surface, enable the active elements to be in direct contact with the base materials own oxide surface, in this case silica, SiO2.
Figure 1. Illustration of ultrasonic soldering process with active solders.
To compare the influence of the active elements on the strength of the soldered bond silica to metal interfaces when using non-activated solders and active solders, a simple overlap soldered coupon ( ~ 1” x 1” ) was used on a compression lap shear test. Figure 2 illustrates the soldered specimen shear test configuration.
Figure 2. Illustration of compression lap shear test configuration.
Table 1 summarizes the lap shear strengths of the active S-Bond® 140 M1 and compares its shear strength to hose of typical non-active element solders.
Table 1. Compression Lap Shear Test Results
The results in Table 1 show that the active S-Bond® 140M1 solders far exceed the shear strengths of non-active elements solders.
To characterize how the active elements are increasing the bond silica-metal joint strengths, Time of Flight – Secondary-ion mass spectrometry [TOF-SIMS] was used to characterize the bond interface. TOF-SIMS measured the distribution of the various S-Bond® elements as a function of depth through the interface. The results show that the activating elements (Ti, Ce, Ga) concentrate at the interface and that their oxides form the interfacial layer between the high purity silica (HPS) and the bulk solder.
The efficacy of these additives was established by demonstrating that the block shear strength of the bond to HFPS was increased by 7 times through the addition of the Ti, Ce and Ga reactive metals to the base Sn-Bi solder.
The resultant data from the investigation showed a significant increase in the concentration of all of the “active” elements present in S-Bond® 140 M1 within a 220 nm interfacial zone between the solder and HPFS.
Figure 3. Charts of Element concentrations made from S-Bond® 140M1 joints between Silica and metal using Time of Flight – Secondary-ion mass spectrometry [TOF-SIMS].
In addition to this accumulation of “active” elements, the quantitative concentration of O was higher in the interfacial region than in areas away from the interface in the solder bulk. These data support the formation of mixed oxides at the interface play a significant role in adhesion. The data also support the notion that the interface comprises an oxide to oxide bond, that is, a silica to active metal oxide bond. All three active elements present in the solder seem to participate relatively equally in this bond formation.
Figure 4. Illustration of mixed oxide bond interface at S-Bond® 140M1 solder to silica surface.
It cannot be necessarily concluded that each active element (Ti, Ce or Ga) has the same contribution to bond strength, or whether having an intermetallic mixed oxide offers an advantage over a simple oxide of a rare earth or titanium. Based on concentration alone, it appears that the role played by all three metals is essentially the same. The thickness of the oxide layer (220 nm) and the observation of an interface layer with mixed oxides supports the model depicted in Figure 4.
The active elements accumulate at the interface because this is the available reaction site due to the presence of the substrate oxide (silica) and potential free oxygen. Once the oxidation reaction occurs, the active metal becomes bound at the interface, and thus accumulates there. The mechanism for movement of the “active” elements to the interfacial region against an apparent concentration gradient is presumably due to mechanical forces, but could also be aided by thermal convection. The ultrasonic energy applied to the system is believed to play a key role in the observed movement of “active” elements to the interfacial region and possibly to an enhanced O level in this same region. Ultrasonic or any other form of mechanical agitation can establish a mixing of the solder that would bring active metal to the interface.
 Nagono, K., Nomaki, K., and Saoyama, Y., US Pat. 3,949,118.
 Ramirez, A.G., Mavoori, H. and Jin, S., “Bonding nature of rare-earth-containing lead-free solders”, Appl. Phys. Lett. 80, 3 21, 398-400: and US Pat. 6,306,516.
 Tomáš Skála, Nataliya Tsud, Kevin C. Prince and Vladimír Matolín, “Bimetallic bonding and mixed oxide formation in the Ga–Pd–CeO2 system”, J. Appl. Phys. 110, 043726 (2011).
 A.R. Lobato, S. Lanfredi, J.F. Carvalho, A.C. Hernandes, “Synthesis, Crystal Growth and Characterization of g-Phase Bismuth Titanium Oxide with Gallium”, Mat. Res. vol.3 n.3 São Carlos July 2000.
This investigation has shown how effective active element solders, such as S-Bond 140M1 are in bonding metals to silica (SiO2) surfaces. If you have applications requiring the bonding and sealing of fused silica or related glasses, please Contact Us… and we can assist in meeting your need.
Fabricating Parts for Proton Collimator With S-Bond® Active Solders® The unique capability of S-Bond solders to join graphite and ceramic to metals was the solution for Los Alamos for fabricating core elements of their Proton Collimator used in its Proton Radiography facility. Conventional brazing was considered but their large differences in Coefficient of Thermal Expansions (CTE’s) limited brazing since on cooling from brazing temperatures (over 800°C), the resultant CTE derived residual stresses would have likely cracked the ceramic, graphite or torn the bond interface. Figure 1 illustrates the graphite – ceramic-stainless steel composite assembly that required stable, thermally and electrical conductive connection between the assembly’s elements.
Los Alamos researchers reached out to S-Bond Technologies to use its S-Bond solders to join these disparate materials. Normally plating would have to be used to make the ceramic and graphite materials solderable. In the case of S-Bond joining, the same solder and soldering process was used to make the joint between the graphite base, the insulating alumina sheet and the stainless steel plate, as depicted in Figure 1.
Figure 1. Illustration of the proton collimator elements joined with S-Bond solders.
The soldering of this composite started the S-Bond metallization of the bonding surfaces of the graphite base and the alumina insulator. In this process, S-Bond metallization paste was applied to the one surface of the graphite and the two opposite sides of the alumina paste. The graphite and the alumina sheet with pastes applied, were heated to 960C in a vacuum furnace in order to react the elements in the paste with the graphite and ceramic surfaces to create a chemical bond between the solder and the graphite and alumina. After metallization these parts’ surfaces are solderable with a well bonded interface. The Graphite base, the alumina insulator plate and the stainless steel header were heated to 250C where S-Bond 220 solder filler metal was applied via melting on and mechanical activation (spreading by heated blade or bush) to pre-tin the faying surfaces of the assembly. Once the S-Bond solder filler metal was pre-placed (pre-tinned) the parts kept hot at 250C, were placed together in an alignment fixture to align the constituent parts accurately and then pressed / loaded with 50 lbs of deadweight as the bonded assembly was cooled.
Figures 2 illustrates the solder bonded composite proton beam collimator component. The pictures show the two S-Bond solder interfaces connecting the water cooled Stainless steel end plate, to the ceramic insulator plate, then connected to the graphite cathode.
Figure 2a. Back of S-Bond joined collimator part. Stainless Steel/ceramic insulator/graphite base (from Top to Bottom)
Figure 2b. Side view of S-Bond joined collimator part.
Figure 3 illustrates the Proton Collimator with the S-Bond joined parts being assembled at Los Alamos National Laboratory (LANL). There were two S-Bond joined parts per assembly. These bonded component assemblies worked very well and enables LANL engineers to successfully implement their design.
Figure 3. Proton Collimator with two S-Bond joined composite being mounted.
LANL engineers were able to utilize S-Bond’s unique capability to solder join stainless steel to ceramic to graphite. If you have such joining challenges, Contact Us for incorporating S-Bond joining in your assemblies.
LED Headlamp Cooling Enabled by Thermal Pyrolytic Graphite (TPG*) Materials via S-Bond Joining
By Dr. Ronald Smith, S-Bond Technologies
Dr. Wei Fan, Momentive Performance Materials Inc.
Automotive LED headlights present a thermal management challenge where conventional aluminum- and copper-based heat sinks limit the maximal power loading to LEDs. Thermal Pyrolytic Graphite (TPG*) materials are now being designed into high power LED headlight assemblies to help thermally manage LED (light emitting diode) heat. TPG* materials contain millions of highly-oriented stacked graphene planes forming excellent in-plane thermal conductivity (>1500 W/m-K) with very low density (2.25 g/cm3). TPG-metal composites can simultaneously achieve high thermal conductivity from the TPG core and high mechanical strength from the metal shell.
In testing conducted by Momentive Performance Materials Inc. (“Momentive”) on prototype headlights, data suggests that replacing aluminum fins with metallized TPG plates can reduce total system thermal resistance by 27%, and by inserting a TPG core underneath LED dies, an additional 24% reduction in thermal resistance can be achieved. Furthermore, testing of this integrated headlight assembly demonstrated that TPG material-assisted heat dissipation at these two strategic locations can allow for a twofold increase in power load to the LED.
Various new bonding processes, including diffusion bonding, epoxy bonding, S-Bond soldering and brazing, have been developed to integrate TPG material into metal-encased TPG-metal composites, such as aluminum, copper, tungsten copper, molybdenum copper and ceramics made through encapsulation processes. TPG-metal composites typically behave like solid metal and can be further machined, plated or bonded to other components to meet various requirements.
TPG-metal composites have been incorporated into Momentive’s TC1050* heat spreaders, TMP-EX heat sinks and TMP-FX thermal straps where they can quickly conduct heat away from the heat-generating sources; therefore, greatly increasing LED cooling efficiency and life. A baseline LED conventional headlight assembly (see Figure 1) shows the LED die bonded to a heat spreader and thermally connected to a cooling fin and active fan system. Thermal analysis of this baseline LED headlight indicated that integrating high-thermal conductivity TPG material into the aluminum heat sink base and aluminum heat sink fins could make the greatest impact in increasing heat dissipation of the LED headlight.
Figure 1. The baseline LED headlight (VLEDS 9006) illustrating key components for thermal management.
Next, TPG materials were integrated into various parts of a LED headlight assembly. In this study, three proposed designs were validated in simulation, built and then tested, as shown in Figure 2.
In Design 1, the fin heat exchanger from the baseline assembly was replaced with straight aluminum fins with similar heat dissipation performance.
Figure 2. New designs with TPG material integrated at heat sink fins and heat sink base to improve heat dissipation. Image: Momentive
In Design 2, the aluminum fins were replaced with Momentive’s TMP-FX thermal straps metallized with S-Bond solder. The high-thermal conductivity TPG material spread the heat more uniformly across the entire fin structure; thus, utilizing the fin surface more efficiently. The thin S-Bond coating not only protected the TPG material from moisture and abrasion, but also enabled soldering of TPG material directly to the aluminum base to minimize any interface heat resistance. Temperature mapping through measurement, as shown in Figure 3, revealed that a thermal resistance of 4.7 oC/W, which was 27% less than the measured resistance of Design 1, was achieved.
Figure 3. Measured LED junction temperature (subtracted by ambient temperature) as a function of input power per LED for the three headlight designs. Note: Test data. Actual results may vary.
In order to facilitate the heat flow from the LED die to the heat sink fins through the narrow neck area, a T-shape TPG tile with 2 mm thickness, as illustrated in Figure 2 as Design 3, was embedded into the aluminum base. This TPG tile, which presented a thermal conductivity 8 times higher than that of aluminum, was first metallized and then brazed into an aluminum enclosure with a T-shape cavity at nearly 600 oC. High temperature braze joints between the TPG material and aluminum and between the aluminum enclosure components provided excellent thermal interfaces and high bonding strength. More importantly, the braze bond endured the downstream S-Bond soldering temperature when the LED dies and TPG fins were attached. This TPG-embedded aluminum heat sink base is a TC1050 heat spreader that can be made via brazing or diffusion bonding processes. With the heat dissipation assist from the TPG tile in the heat sink base, in combination with the S-Bond-joined heat sink fins, the measured thermal resistance of Design 3 was 3.0 oC/W, another 24% reduction as compared to Design 2 (see Figure 3).
Thermal simulation conducted on Designs 1, 2 and 3 with input power of 15 W per LED (30 W total) is illustrated in Figure 4. The simulation results showed that the TPG S-Bond soldered fins in Design 2 reduced the temperature gradient along the fins; hence, increasing the effective area dissipating heat to the air circulation. In the case of Design 3, the T-shape TPG tile in the heat sink base further decreased the temperature gradient between the LED die to the heat sink fins. Compared with Design 1, the combination of the TPG heat sink fins and TPG heat sink base in Design 3 allowed a significantly lower LED temperature.
Figure 4. Simulated temperature profiles of LED headlights with Design 1, 2 and 3, respectively. Image: Momentive
These studies, conducted by Momentive on its TPG materials and components, have shown that S-Bond metallization and soldering presents a key element to the manufacture of LED headlamps with improved cooling.
If you want to learn more about joining TPG materials, or to otherwise fabricate thermal management components, please Contact Us at S-Bond Technologies or Contact Momentive for data and information on their TPG materials and/or devices.
*TPG and TC1050 are trademarks of Momentive Performance Materials Inc.
Carbon/graphite compressor seal rings are employed in many compressors and more and more metal backed graphite seals are being used in higher efficiency compressors. Frictional heating of seals can degrade metal backed graphite seals, therefore good thermal contact between the graphite seal ring and the metal backing is needed to improve cooling of the seal. S-Bond Technologies has developed active soldering methods for graphite bonding which is now being used to manufacture rotating metal backed graphite seals.
In the process, graphite rings are initially S-Bond metallized to create a chemically bonded solder to graphite interface. The sequence of bonding is shown in the figures below. After metallization the metal rings’ bonding surface are pre-tinned with S-Bond filler metals, via heating, melting of the solder surfaces on the metal followed by mechanical activation (ultrasonic solder tip agitation). The metallized graphite ring surface, at 250C is pressed against the pre-tinned metal backing ring surface and then the assembled ring is cooled to solidify the solder joint.
These S-Bond solder joined graphite to metal backing ring seals have endured 1,000’s of hours of running in natural gas compressors and are providing the customer with improved graphite – metal backed ring seal performance.
Please Contact Us for your graphite to metal bonding solutions.
Copper has superior cooling capacity than aluminum and is the preferred heat sink material for telecommunications and high power electronics. However, the weight and cost or copper limits the size of the heat sink packages. Therefore for larger electronic enclosures a hybrid design, using copper for a localized heat sink joined to an aluminum frame with good thermal contact can significantly improve the cooling performance of a heat sink package.
Joining copper to aluminum poses it challenges. Cu and Al cannot be welded easily due to the intermetallics that form when Cu alloys with Al in the weld pool. Alternatively, brazing cannot be done since melt point of aluminum is below the typical Cu-Ag braze filler metals (silver solders) used to braze copper. These issues leave “soldering” as the metal filler joining process of choice. But alone, soldering of Cu to Al has challenges. Solders, typically Sn-Ag based, cannot easily wet and adhere to aluminum without first plating the aluminum with nickel or using very aggressive chemical fluxes which themselves are incompatible with soldering to copper.
S-Bond Technologies, working with its customers has demonstrated its active solder, S-Bond 220-50, join Cu to Al in all configurations. The figures below show an example of where a copper- finned heat sink assembly was S-Bond joined into a finned aluminum package. In this assembly, the Cu-fins were individually S-Bond soldered into copper heat sink base, after which the Cu fin-base assembly was then S-Bond joined into the aluminum base at 250C. This soldering temperature well below the softening temperatures for the aluminum frame and low enough that the thermal expansion mismatch between Cu and Al did not distort the bonded assembly when cooling.
Hybrid heat sinks, combing the thermal benefits of copper with lightweight aluminum are taking advantage of the capabilities of active solder joining. For tough dissimilar materials and copper and aluminum bonding challenges, Contact us.
S-Bond Technologies’ active solder joining solutions have been used by by Fermi National Accelerator Laboratory for joining carbon:carbon and pyrolytic graphite in its particle accelerator program. The improved Forward Particle Detector (FPIX) is to be used in Compact Muon Solenoid (CMS) and used for high-resolution, 3D tracking points, essential for pattern recognition and precise vertexing, all embedded in a hostile radiation environment.
The challenge posed by Fermi Lab to S-Bond Technologies, was to create high thermal conductivity metallic bonds between the ends of thermally pyrolytic graphite (TPG) blades and carbon:carbon composite end walls of a turbine nozzle like assembly. Figure 1(a-b) shows ¼ of a full assembly that was built using S-Bond joining and S-Bond 220 solder and processing. S-Bond’s active solder processing was successfully demonstrated and is in the technical roadmap for assembling Fermi Lab’s particle accelerator FPIX. In this work, regular adhesives were not conductive and would off-gas in the extremely high vacuum environments, hence the selection of the active metal solder filler, S-Bond 220.
Figures 2 – 4 show various assembly steps that all utilize the S-Bond carbon:carbon joining process described elsewhere in our technical blog. The process started with the S-Bond metallization on the TPG blades, Figure 2, and the C:C nozzle endwall slots
where the blades were inserted, Figure 3. After metallization, the blade and nozzle segments were inserted into a heated alignment press, Figure 4. After heating and insertion, the nozzle segment/ TPG blade assembly was cooled and removed. Figures 1a – b, above, showed the final fully bonded assembly.
Please contact us if you have challenging graphite / carbon joining applications where S-Bond Technologies active solders can solve your joining problems.